Just to clarify: an infanta or anyone else in the line of succession can renounce their rights to succeed to the Crown of Spain, but all renunciations , like any abdication of a monarch, has to be ratified by an organic law, which is a special type of act of parliament that requires, if I am not mistaken, an absolute majority in the Congress of Deputies to be passed.
In other words, Cristina could give up her succession rights, but the renunciation would have to be handled in the same way as her father’s abdication was.
On the issue of royal marriages, the Spanish monarchy is actually slightly different from its European counterparts. In other countries, succession rights are forefeited if a royal marries without consent., so a royal marriage has to be explicitly consented to after consent is asked. In Spain, it is not necessary to ask for and receive consent to a royal marriage; instead, it suffices that the King or the parliament do not explicit prohibit the intended marriage.
The relevant references for all of the points above, which are related to previous posts in this forum, is Art. 57 of the Soanish constitution.